Sadly, after taking my tree out of the greenhouse due to extreme heat, several of my trees got leaf burn....areas or streaks on the top leaves that have turned brown in color. Is there anything I can do for the leaves? Should I leave them on the tree or just let nature take care of them?

I would let nature take care of them. The leaves might still be supplying photosynthesis. When a citrus leaf becomes a deficit to the tree, the tree will quickly drop the leaf. There is no welfare program for citrus leaves. - Millet (980-)

What causes leaf burn if the tree is supposed to be fine in full sunlight? Thanks, Tim.

Citrus leaves get adjusted to lower light, then when they are suddenly exposed to full sun it causes a sunburn--just like we get unless we are adapted to full sun. Citrus reach their maximum photosynthesis rate(Pmax) at about 650 PAR or about 1/3 full sunlight. When they are adapted to full sun they have smaller cloroplast and more acessory pigments to help protect the leaf from full sun.

So what do I need to do to keep my trees from getting leaf burn? I took them out of the greenhouse due to extreme temperatures and now I have them all outside. Should I have moved them to partial sun first? I would have thought that they would have been used to full sun by being in the greenhouse? Thanks, Tim.

UV light which does the burning, does not make it thru a lot of materials.

Yes, acclimate in partial shade first.